Electric motor

ABSTRACT

An electric motor includes a stator including four split cores, and a rotor having four magnetic poles. Each of the split cores includes a yoke and a tooth. An angle θ 1  [degree] formed by a side surface of the tooth and a side surface of the yoke on an inner side in a radial direction of the stator satisfies 90 degrees≤θ 1&lt;180  degrees.

Cross Reference to Related Application

This application is a U.S. national stage application of InternationalPatent Application No. PCT/JP2017/006979 filed on Feb. 24, 2017, thedisclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an electric motor.

BACKGROUND

A stator core formed of a plurality of split cores is typically used inorder to ease winding of a wire around a stator of an electric motor.Easy winding of the wire around the stator can enhance the density of astator coil and increase motor efficiency. In an electric motordisclosed in Patent Reference 1, for example, a stator core is dividedinto twelve core elements, and thus the electric motor includes twelveteeth.

PATENT REFERENCE

Patent Reference 1: Japanese Patent Application Publication No.2005-117844

In general, however, as the number of magnetic poles and the number ofteeth of the stator increase, the electrical frequency of a currentinput to the electric motor increases. Accordingly, the waveform of thecurrent input to the electric motor becomes roughened and consequentlycontrollability of the electric motor (e.g., rotor rotation control)deteriorates. Thus, to enhance controllability of the electric motor inhigh-speed rotation at 10000 rpm or more, for example, the number ofmagnetic poles and the number of teeth are preferably as small aspossible. For this reason, there has been a demand for an electric motorthat includes a small number of magnetic poles and a small number ofteeth and includes a stator including split cores around which a wire iseasily wound.

SUMMARY

It is therefore an object of the present invention to provide anelectric motor including a stator around which a wire is easily wound,and having high controllability.

An electric motor according to present invention includes: a statorincluding four split cores; and a rotor disposed inside the stator andhaving four magnetic poles. Each of the split cores includes a yoke anda tooth. The yoke includes a joint part having a length from the toothoutward in a radial direction of the stator and a back yoke having alength from the joint part inward in the radial direction. An angle θ1[degree] formed by a side surface of the tooth and a side surface of theyoke on an inner side in the radial direction of the stator satisfies 90degrees≤θ1<180 degrees.

The present invention can provide an electric motor in which a wire iseasily wound around a stator and having high controllability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view schematically illustrating a structure of anelectric motor according to an embodiment of the present invention.

FIG. 2 is a plan view schematically illustrating a structure of a splitcore.

FIG. 3 is a plan view schematically illustrating the structure of thesplit core.

FIG. 4 is a plan view schematically illustrating the structure of thesplit core.

FIG. 5 is a plan view schematically illustrating a structure of theelectric motor including a stator fixed by caulked parts.

FIG. 6 is a plan view schematically illustrating a structure of a splitcore including the stator core fixed by caulked parts.

FIG. 7 is a diagram showing a relationship between a rotation speed[rps] of the electric motor and an electrical frequency [Hz] of acurrent input to the electric motor, for each number of magnetic polesof a rotor.

FIG. 8 is a diagram showing a waveform of a current input to theelectric motor according to the embodiment.

FIG. 9 is a diagram showing a waveform of a current input to an electricmotor having eight magnetic poles as a comparative example.

FIG. 10 is a plan view schematically illustrating a structure of anelectric motor as a first comparative example.

FIG. 11 is a plan view schematically illustrating a structure of a splitcore of the electric motor as the first comparative example.

FIG. 12 is a view illustrating an example of a process of winding a wirearound the split core of the electric motor as the first comparativeexample.

FIG. 13 is a plan view schematically illustrating a structure of anelectric motor as a second comparative example.

FIG. 14 is a plan view schematically illustrating a structure of a splitcore of the electric motor as the second comparative example.

FIG. 15 is a plan view schematically illustrating a structure of anelectric motor as a third comparative example.

FIG. 16 is a plan view schematically illustrating a structure of a splitcore of the electric motor as the third comparative example.

FIG. 17 is a plan view illustrating the electric motor according to thethird comparative example disposed in a frame.

FIG. 18 is a plan view illustrating the electric motor according to theembodiment disposed in a frame.

DETAILED DESCRIPTION Embodiment

FIG. 1 is a plan view schematically illustrating a structure of anelectric motor 1 according to an embodiment of the present invention.

FIGS. 2 through 4 are plan views schematically illustrating a structureof a split core 20.

The electric motor 1 includes a stator 2, a rotor 3, and a positionsensor 4. The electric motor 1 is, for example, a permanent magnetsynchronous motor.

The electric motor 1 is driven by, for example, a single-phase inverter.In the case where the electric motor 1 is driven by the single-phaseinverter, the number of times of switching can be reduced as compared toa case where a three-phase inverter is used, for example, and aswitching loss during high-speed rotation can be reduced. Duringhigh-speed rotation, an electrical frequency increases and the number oftimes of switching increases. Thus, the advantage of using asingle-phase inverter can be obtained especially during high-speedrotation.

In the case where the number of times of switching is small in theinverter, the waveform of a current input to the electric motor 1 isstrained, and iron loss in the stator 2 increases due to a harmoniccomponent of this current. Thus, as a material for a stator core 21 ofthe stator 2, a material such as amorphous metal is preferably usedinstead of an electromagnetic steel sheet. This can reduce occurrence ofan iron loss in the stator 2 and also suppress degradation of motorefficiency.

The stator 2 includes the stator core 21, an insulator 22, and aplurality of split faces 23. The rotor 3 is disposed inside the stator 2with an air gap interposed therebetween. The insulator 22 insulates thestator core 21. A wire is wound around the stator core 21 with theinsulator 22 interposed therebetween.

The stator core 21 includes a plurality of yokes 21 a and a plurality ofteeth 21 b. The stator core 21 is formed by, for example, laminating aplurality of amorphous metal sheets or a plurality of electromagneticsteel sheets.

The insulator 22 is disposed in a slot formed between adjacent two ofthe teeth 21 b. Specifically, the insulator 22 is fixed to a sidesurface of the stator core 21. The insulator 22 is made of, for example,insulating resin.

An outer peripheral surface 24 of the stator core 21 is formed in theshape of a circular arc. Specifically, the outer peripheral surface 24is an outer peripheral surface of the yoke 21 a formed at an outermostportion of the yoke 21 a in the radial direction.

The rotor 3 includes a plurality of permanent magnets and a plurality ofmagnetic poles. In this embodiment, the rotor 3 has four magnetic poles.

The position sensor 4 includes a Hall element for detecting a magneticfield from the rotor 3, for example. The position sensor 4 is fixed bythe insulator 22 beside the tooth 21 b in a circumferential direction.Specifically, the position sensor 4 is fixed by the insulator 22 betweenadjacent two teeth 21 b. Accordingly, the size of the electric motor 1can be reduced.

The electric motor 1 can be easily controlled by detecting the magneticfield from the rotor 3 by using the position sensor 4 and by detecting arotation position (phase) of the rotor. In addition, since the positionsensor 4 is fixed between two teeth 21 b, it is possible to prevent theelectric motor 1 from increasing in size and to make the electric motor1 small in size.

A structure of the split core 20 will now be described.

An arrow D1 in FIGS. 2 through 4 represents a circumferential directionof the stator 2, the stator core 21, and the rotor 3 (hereinafter alsoreferred to as simply a “circumferential direction”). An arrow D2 inFIGS. 2 through 4 represents a radial direction of the stator 2, thestator core 21, and the rotor 3 (hereinafter also referred to as simplya “radial direction”). In FIGS. 2 through 4, an arrow D21 represents aninner side in the radial direction, and an arrow D22 represents an outerside in the radial direction.

The stator 2 includes the plurality of split cores 20. In thisembodiment, the stator 2 is formed of four split cores 20.

The stator 2 is divided into parts (i.e., four split cores 20) in thesame number as the teeth 21 b. Accordingly, the stator 2 includes fouryokes 21 a and four teeth 21 b.

Each of the split cores 20 includes the stator core 21 and the insulator22. Each stator core 21 includes one yoke 21 a, one tooth 21 b, and twosplit faces 23. Each of the split faces 23 is formed at an end portionof the yoke 21 a of the stator core 21 in the circumferential direction.A split end portion 23 a is an end portion of each of the split faces 23on the inner side in the radial direction.

Each yoke 21 a includes a back yoke 211 and a joint part 212. The jointpart 212 has a length from the tooth 21 b outward in the radialdirection. The back yoke 211 has a length extending from the joint part212 inward in the radial direction. The tooth 21 b extends inward in theradial direction.

An angle θ1 [degree] formed by a side surface 21 c of the tooth 21 b anda side surface 21 d of the yoke 21 a on the inner side in the radialdirection satisfies 90 degrees≤θ1<180 degrees. In the exampleillustrated in FIG. 2, the side surface 21 c of the tooth 21 b is asurface extending in the radial direction, that is, a surface at eachside of the tooth 21 b in a direction orthogonal to the radialdirection. The side surface 21 d of the yoke 21 a is adjacent to theside surface 21 c of the tooth 21 b.

In addition, as illustrated in FIG. 3, an angle θ1 [degree] preferablysatisfies 90 degrees≤θ1<180 degrees. This can ease winding of the wirearound the tooth 21 b.

Similarly, as illustrated in FIG. 3, an angle θ2 [degree] formed by aside surface 22 a of the insulator 22 fixed to the tooth 21 b and a sidesurface 22 b of the insulator 22 fixed to the yoke 21 a satisfies 90degrees≤θ2<180 degrees. The side surface 22 b is adjacent to the sidesurface 22 a.

The angle θ2 [degree] preferably satisfies 90 degrees<θ1<180 degrees.This can ease winding of the wire around the tooth 21 b.

As illustrated in FIG. 4, each split end portion 23 a is located outsidea line L1 in the radial direction. The line L1 is a boundary between theyoke 21 a and the tooth 21 b. That is, the line L1 is a boundary betweenthe side surface 21 d of the yoke 21 a and the side surface 21 c of thetooth 21 b.

Similarly, an end portion 22 c that is an end portion of the insulator22 in the circumferential direction is located outside the line L1 inthe radial direction. Accordingly, an advantage of easing wiring of thewire can be obtained, and the density of the stator coil can beincreased.

FIG. 5 is a plan view schematically illustrating a structure of theelectric motor 1 including the stator 2 fixed by caulked parts 25.

FIG. 6 is a plan view schematically illustrating a structure of thesplit core 20 including the stator core 21 fixed by caulked parts 25.

As illustrated in FIGS. 5 and 6, a plurality of amorphous metal sheetsor a plurality of electromagnetic steel sheets forming the stator core21 may be fixed by caulked parts 25. The structure of the electric motor1 illustrated in FIG. 5 and the split core 20 illustrated in FIG. 6 arethe same as those of the electric motor 1 and the split core 20,respectively, illustrated in FIGS. 1 through 4 except for the caulkedparts 25.

The use of the caulked parts 25 stabilizes the shape of the stator core21 to thereby stabilize the shape of the split faces 23. Accordingly, infitting a frame 5 (FIG. 18) into the electric motor 1 by shrink fitting,for example, it is possible to prevent the split cores 20 from becomingapart from one another at the split faces 23.

Advantages of the electric motor 1 according to this embodiment will nowbe described.

FIG. 7 is a diagram showing a relationship between the rotation speed[rps] of the electric motor and an electrical frequency [Hz] of acurrent input to the electric motor, for each number of magnetic polesof a rotor. Specifically, FIG. 7 is a diagram showing a relationshipbetween the rotation speed of the electric motor and the electricalfrequency in the case where the number of magnetic poles of the electricmotor is changed. In FIG. 7, f1 indicates a relationship between therotation speed and the electrical frequency in the electric motor 1according to this embodiment, f2 indicates a relationship between therotation speed and the electrical frequency in an electric motorincluding six magnetic poles and six teeth, f3 indicates a relationshipbetween the rotation speed and the electrical frequency in an electricmotor including eight magnetic poles and eight teeth, and f4 indicates arelationship between the rotation speed and the electrical frequency inan electric motor including ten magnetic poles and ten teeth.

FIG. 8 is a diagram showing a waveform of a current input to theelectric motor 1 according to this embodiment.

FIG. 9 is a diagram showing a waveform of a current input to an electricmotor including eight magnetic poles as a comparative example. Thewaveforms shown in FIGS. 8 and 9 are waveforms of a current input to theelectric motor 1 while the rotor 3 makes one turn, and have the samecarrier frequency.

As shown in FIG. 7, as the number of magnetic poles increases, theelectrical frequency of the current input to the electric motorincreases. For example, when the number of magnetic poles increases fromfour to eight, the electrical frequency doubles. Thus, as shown in FIG.9, the waveform of a current input to the electric motor including eightmagnetic poles is rougher than that in the electric motor 1 includingfour magnetic poles (FIG. 8) when the carrier frequency is constant. Asthe waveform of a current becomes rougher, controllability of theelectric motor (e.g., rotation control of a rotor) deteriorates. Thus,to drive the electric motor in high-speed rotation of 10000 rpm or more,the number of magnetic poles is preferably as small as possible and theelectrical frequency is preferably as low as possible. Then,controllability of the electric motor is improved.

As described above, to improve controllability of the electric motor 1,in the electric motor 1 according to this embodiment, the rotor 3includes four magnetic poles and the stator 2 includes four teeth 21 b.Accordingly, as compared to an electric motor including six or moremagnetic poles, controllability can be enhanced even in the case ofdriving at 10000 rpm or more.

In the case where the electric motor 1 is driven by a single-phaseinverter, the number of times of switching can be reduced and aswitching loss in high-speed rotation can be reduced, as compared to thecase of driving by a three-phase inverter, for example.

Each yoke 21 a includes a back yoke 211 and a joint part 212. The jointpart 212 has a length from the tooth 21 b outward in the radialdirection. The back yoke 211 has a length from the joint part 212 inwardin the radial direction. Accordingly, a region where a stator coil isformed by winding of the wire can be enlarged.

FIG. 10 is a plan view schematically illustrating a structure of anelectric motor 1 a as a first comparative example.

FIG. 11 is a plan view schematically illustrating a structure of a splitcore 20 a of the electric motor 1 a as the first comparative example.

FIG. 12 is a view illustrating an example of a process of winding a wire7 around the split core 20 a.

The electric motor 1 a according to the first comparative exampleincludes four magnetic poles and four teeth 121 b, in a manner similarto the electric motor 1 according to this embodiment. The electric motor1 a includes four split cores 20 a. The teeth 121 b correspond to theteeth 21 b of the electric motor 1 and are the same as the teeth 21 b instructure. In the electric motor 1 a, a yoke 121 a differs in structurefrom the yoke 21 a of the electric motor 1 according to the embodiment.Specifically, a maximum angle θ3 formed by a side surface 121 c of thetooth 121 b and a side surface 121 d of the yoke 121 a is smaller than90 degrees.

A wire is typically wound around the tooth using a nozzle. If the statoris not divided into a plurality of cores, it is difficult to wind a wireso as to increase the density of a stator coil. On the other hand, inthis embodiment, since the stator 2 is divided into a plurality ofcores, the wire can be easily wound around each of the teeth 21 b usingthe nozzle, and the density of the stator coil can be increased. Toprevent a positional shift of the position sensor 4, the wire is notpreferably wound near the position sensor 4.

In the electric motor la illustrated in FIGS. 10 and 11, however, sincethe maximum angle θ3 is smaller than 90 degrees, the split end portion23 a is located inside the line L1 in the radial direction asillustrated in FIG. 12. Accordingly, the yoke 121 a hinders operation ofa nozzle 6, and it is difficult to wind a wire 7 especially around aportion of the tooth 121 b on the outer side in the radial direction. Onthe other hand, in the electric motor 1 according to this embodiment,the angle θ1 [degree] satisfies 90 degrees≤θ1<180 degrees. Accordingly,in the electric motor 1 according to this embodiment, the split core 20can be formed such that the split end portion 23 a is located outsidethe line L1 in the radial direction, and thus, the wire can be easilywound, advantageously.

FIG. 13 is a plan view schematically illustrating a structure of anelectric motor lb as a second comparative example.

FIG. 14 is a plan view schematically illustrating a structure of a splitcore 20 b of the electric motor 1 b as the second comparative example.

The electric motor 1 b according to the second comparative exampleincludes two magnetic poles and two teeth 221 b. The electric motor 1 bincludes two split cores 20 b. In the electric motor 1 b, a yoke 221 adiffers in structure from the yoke 21 a of the electric motor 1according to the embodiment. Specifically, a maximum angle θ4 formed bya side surface 221 c of the tooth 221 b and a side surface 221 d of theyoke 221 a on the inner side in the radial direction is smaller than 90degrees.

Thus, in a manner similar to the electric motor la according to thefirst comparative example, since the maximum angle θ4 is also smallerthan 90 degrees in the electric motor 1 b, the yoke 221 a hindersoperation of the nozzle 6 in winding a wire, and it is difficult to winda wire especially around a radially outer portion of the tooth 221 b.

As described with reference to FIG. 7, in view of controllability of theelectric motor 1, it is desirable that the number of magnetic poles andthe number of teeth should be smaller. On the other hand, in view ofwinding of a wire, each of the number of magnetic poles and the numberof teeth is preferably four. Even in the case where each of the numberof magnetic poles and the number of teeth is four, as in the electricmotor 1 a according to the first comparative example, if the maximumangle θ3 is smaller than 90 degrees, winding of the wire is difficult.

Thus, in this embodiment, the electric motor 1 includes four magneticpoles and the four teeth 21 b, and the angle θ1 [degree] satisfies 90degrees≤θ1<180 degrees. Accordingly, controllability of the electricmotor 1 can be enhanced, and winding of the wire can be made easy,advantageously. Thus, the density of the stator coil can be enhanced.

In addition, in this embodiment, in the yoke 21 a, the joint part 212has a length from the tooth 21 b outward in the radial direction. Theback yoke 211 has a length from the joint part 212 inward in the radialdirection. Accordingly, each of the split cores 20 can be formed suchthat the split end portion 23 a is located outside the line L1 in theradial direction, and thus, winding of the wire can be made easy.

The position sensor 4 is fixed by the insulator 22 between each adjacenttwo of the teeth 21 b. Accordingly, the size of the electric motor 1 canbe reduced.

FIG. 15 is a plan view schematically illustrating a structure of anelectric motor 1 c as a third comparative example.

FIG. 16 is a plan view schematically illustrating a structure of a splitcore 20 c of the electric motor 1 c as the third comparative example.

The electric motor 1 c according to the third comparative exampleincludes four magnetic poles and four teeth 321 b, in a manner similarto the electric motor 1 according to this embodiment. The electric motor1 c includes four split cores 20 c. In the electric motor 1 c, a yoke321 a differs in structure from the yoke 21 a of the electric motor 1according to the embodiment. Specifically, the yoke 321 a is formedstraight. In the electric motor 1 c, an angle θ5 formed by a sidesurface 321 c of the tooth 321 b and a side surface 321 d of the yoke321 a on the inner side in the radial direction is 90 degrees. Thus, ina manner similar to the electric motor 1 according to this embodiment,controllability of the electric motor 1 c can be enhanced, and windingof the wire can be made easy, advantageously.

FIG. 17 is a plan view illustrating the electric motor 1 c according tothe third comparative example disposed in the frame 5.

FIG. 18 is a plan view illustrating the electric motor 1 according tothe embodiment disposed in the frame 5.

The frame 5 is a cylindrical frame. As illustrated in FIG. 17, in theelectric motor 1 c, in the case where a contact portion 24 c in contactwith the inner peripheral surface of the frame 5 is in point contactwith the frame 5, fixing of the stator 2 c (four split cores 20 c) isnot stable in the frame 5, and the shape of the stator 2 c is not easilymaintained.

As illustrated in FIG. 18, in the electric motor 1 according to theembodiment, the contact portions in contact with the inner peripheralsurface of the frame 5 are outer peripheral surfaces 24 of the yokes 21a (specifically, the back yokes 211) formed in the shape of a circulararc. Since the outer peripheral surfaces 24 have a shape of a circulararc, the outer peripheral surface 24 is in surface contact with theframe 5. Accordingly, fixing of the stator 2 is stabilized in the frame5 and consequently the shape of the stator 2 can be easily maintained,advantageously.

Features of the embodiment and features of the comparative examplesdescribed above can be combined as appropriate.

1. An electric motor comprising: a stator including four split cores;and a rotor disposed inside the stator and having four magnetic poles,wherein each of the split cores includes a tooth, and a yoke including ajoint part having a length from the tooth outward in a radial directionof the stator and a back yoke having a length from the joint part inwardin the radial direction, and an angle θ1 [degree] formed by a sidesurface of the tooth and a side surface of the yoke on an inner side inthe radial direction of the stator satisfies 90 degrees≤θ1<180 degrees.2. The electric motor according to claim 1, wherein each of the splitcores includes a split face formed at an end portion of the split corein a circumferential direction of the stator, and an end portion of thesplit face on the inner side in the radial direction is located outsidea boundary between the yoke and the tooth in the radial direction. 3.The electric motor according to claim 1, wherein the angle θ1 [degree]satisfies 90 degrees<θ1<180 degrees.
 4. The electric motor according toclaim 1, wherein an outer peripheral surface of the back yoke is formedin a shape of a circular arc.
 5. The electric motor according to claim1, further comprising a position sensor to detect a magnetic field fromthe rotor.
 6. The electric motor according to claim 5, furthercomprising an insulator insulating the tooth, wherein the positionsensor is fixed by the insulator beside the tooth in a circumferentialdirection.
 7. The electric motor according to c1aim 1, wherein theelectric motor is driven by a single-phase inverter.
 8. The electricmotor according to c1aim 1, wherein the electric motor is driven at10000 rpm or more.